Effect of Network Density in Surface-Anchored Poly(N-isopropylacrylamide) Hydrogels on Adsorption of Fibrinogen

We describe a simple approach to generate surface-attached biocompatible hydrogels with tunable cross-link density and employ them to study the effect of gel structure on protein adsorption. Using free-radical polymerization, we synthesize a series of random copolymers comprising N-isopropylacrylamide (NIPAm) and the photoactive curing agent 4-methacryloyl-oxy-benzophenone (MABP) of mole fractions ranging from 2.5 to 10%. We deposit a thin film of the precursor copolymer (∼150 nm) on a silicon or glass substrate, which is precoated with monolayers of benzophenone-silane, then cross-link it through UV irradiation at 365 nm (dose ≈ 6-10 J/cm²) to generate surface-attached networks. A systematic investigation of the network properties such as gel fraction, cross-link density, and swelling ratio reveals that gels with higher MABP content (≥5%) produce densely cross-linked hydrophobic networks with low or no swelling in an aqueous medium. We study the adsorption of fibrinogen (Fg) on such hydrogel substrates and establish that the amount of adsorbed Fg depends on the degree of cross-linking and the swelling capacity of the networks. Specifically, although Fg adsorbs heavily on denser networks, loosely bound gels that swell in aqueous medium repel proteins. We attribute the latter behavior to entropic shielding and size-exclusion factors.